The objectives of this work are to 1) investigate how barbiturate self-administration is altered by the development of tolerance to barbiturates and more specifically, to determine whether the reinforcing property of a barbiturate is altered by the development of tolerance to this drug; and 2) to investigate and compare the stimulus properties of pentobarbital in rhesus monkeys and pigeons. In addition, studies of the development of tolerance to the discriminative properties of barbiturates will be conducted, and changes in the sensitivity of the animals to pentobarbital and bemegride will be evaluated following induction of tolerance.